Problem: Solve for $r$ : $8 = r + 13$
Explanation: Subtract $13$ from both sides: $ \begin{eqnarray} \\ \hphantom{8 {- 13}} &\hphantom{=}& \hphantom{r} \\ 8 &=& r + 13 \\ \\ {-13} && {-13} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 8 {- 13} &=& r \\ \end{eqnarray} $ Simplify. $ r = -5$